This invention relates to method and apparatus for voice encryption and in particular to a new and improved voice encryption system which is small enough to be portable, while being reliable and having a very high probability that the encryption cannot be broken in a reasonable amount of time. While a voice signal is referred to in the specification and claims, it will be understood that the system is applicable to any audio signal; the voice signal is referred to in the discussion because voice encryption is the most common usage for such systems.
In a typical encryption system, the voice signal is encoded or converted to another signal which is then transmitted to a receiver where it is decoded or reconverted back to the original voice signal. The signal transmission may be by various means and the present invention will be described in conjunction with a conventional telephone transmission line.
One type of encryption system utilized in the past is the frequency scrambling system wherein the audio signal is connected as an input to each of a bank of analog filters which function to separate the analog signal into a plurality of frequency segments. In the encoder, the sequence of the segments is transposed or swapped to higher or lower frequency bands and the resultant analog signals are combined for transmission. In the decoder, the reverse of the operation is performed to reproduce the original voice signal. This system is not satisfactory because analog filters have limitations in frequency resolution which permit only a few frequency segments to be used. By way of example, with the standard telephone circuit a bandwidth of 2560 Hertz is used to carry the voice information. An analog filter system in this bandwidth cannot utilize more than 5 to 10 frequency bands. A five bank system provides only 120 possible frequency transpositions or scrambling combinations. A ten bank system provides about 3.times.10.sup.6. While this number of possible combinations would provide some security, operation with ten analog filters in the standard telephone bandwidth is exceedingly difficult.
Accordingly, it is an object of the present invention to provide a new improved voice encryption method and apparatus which can utilize many more frequency bands and provide a very high number of scrambling combinations. By way of example, a system with 16 frequency bands or segments will provide 2.times.10.sup.13 possible combinations and a system with 64 segments will provide 10.sup.89 possible combinations.
The previously described prior art system is all analog. In another prior art encryption system, signals are digitally processed in various ways providing a digital output for transmission. For a high quality encryption system, a relatively wide transmission bandwith is required.
It is an object of the present invention to provide a new and improved method and apparatus for voice encryption which can handle a variety of audio inputs resulting from various speakers utilizing different languages, accents and dialects, and which provides an analog output with narrow bandwidth capability, while at the same time utilizing digital processing.
It is also an object of the invention to provide a means of changing the transposing or swapping order at frequent intervals during a voice transmission to provide a different sequence. The security of the voice communication is greatly enhanced by changing the swapping order several times a second, preferably at least 5 times a second. This greatly disguises audible patterns in the cipher, which are very useful in intercepting the communication. These and other objects, advantages, features and results will more fully appear in the course of the following description.